1. Field of the Invention
The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for ensuring that the pod door is firmly and securely retained on the port door as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Parikh and Uhrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called xe2x80x9cbottom openingxe2x80x9d pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide xe2x80x9cfront openingxe2x80x9d pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell. For both front opening and bottom opening pods, a pod door includes a front surface which is included as part of the sealed pod environment, and a rear surface which is exposed to the environment of the wafer fab.
In order to transfer wafers between a SMIF pod and a process tool within a wafer fab, a pod is typically loaded either manually or automatedly onto a load port on a front of the tool. The process tool includes an access port which, in the absence of a pod, is covered by a port door which includes a front surface exposed to the wafer fab environment and a rear surface which is part of the sealed environment within the process tool. The SMIF pod is seated on the load port so that the pod door and port door lie adjacent to each other. Registration pins are provided on the port door that mate with grooves in the pod door to assure a proper alignment of the pod door with respect to the port door.
Once the pod is positioned on the load port, mechanisms within the port door unlatch the pod door from the pod shell and move the pod door and port door together into the process tool where the doors are then stowed away from the wafer transfer path. The pod shell remains in proximity to the interface port so as to maintain a clean environment including the interior of the process tool and the pod shell around the wafers. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod for transfer between the pod and the process tool.
It is extremely important to provide a clean, low particulate and contaminant environment around the exposed wafers within the process tool. While the air within wafer fabs is typically filtered to some degree, the environment surrounding the process tools and SMIF pods include relatively high levels of particulates and contaminants as compared to within the pods and tools. As such, significant steps are taken to isolate SMIF pod and process tool interiors from the surrounding environment within the fab.
As explained above, the pod door and port door, even though having surfaces exposed to the environment of the wafer fab, are typically brought into the interior of the process tool in preparation for wafer transfer between the pod and the tool. In order to prevent the particulates and contaminants on the exposed door surfaces from contaminating the interior of the process tool, it is known to hold the exposed pod and port door surfaces against each other when bringing the pod and port doors into the process tools and while the doors are positioned therein. Such contact may trap particulates and/or contaminants between the exposed surfaces to thereby prevent the transfer of the particulates and/or contaminants into the process tool.
Coupling mechanisms are known for coupling the pod door to the port door as the pod door is removed from the pod and stowed in the process tool. However, without additional restraints between the pod and port doors, it is possible that the pod door will vibrate on the port door, or that the pod door will tilt or otherwise move with respect to the port door. Any such vibration or movement may result in particulates and/or contaminants dislodging from the pod and/or port door surfaces and settling in the process tool.
Prior art attempts have been made to hold the pod door firmly against the port door while the doors are coupled together and stowed in the process tool. One such system is disclosed in U.S. Pat. No. 5,772,386, entitled xe2x80x9cLoading and Unloading Station for Semiconductor Processing Installationsxe2x80x9d, which patent is assigned to Jenoptik A. G. As set forth therein, the port door may include a pair of suction cups connected to a vacuum source. When the pod door is coupled to the port door, the suction cups engage a surface of the pod door, and the vacuum source creates suction within the cups to hold the pod door to the port door. In addition to the fact that particulates and contaminants may still escape from between the doors into the process tool, there is a further disadvantage to the disclosed system in that the vacuum source may fail or that the suction between the pod and port door may otherwise be lost. In such an instance, the pod door would potentially be able to vibrate or move around with respect to the port door as explained above. Moreover, this type of system requires a fab to provide a vacuum source as an additional utility to the process tool. Not only does this increase the cost and complexity of the tool design, but the control system must also include routines for monitoring the vacuum source to ensure proper operation.
It is therefore an advantage of the present invention to minimize the amount of particulates and contaminants that may dislodge from between the pod and port doors in the process tool.
It is another advantage of the present invention to provide a tight seal between the port and pod doors when the doors are latched together to prevent the escape of contaminants and/or particulates from between their juxtaposed surfaces.
It is a further advantage of the present invention to establish a tight seal between the port and pod doors entirely by mechanisms within the port and pod doors, without requiring additional monitors and/or utilities such as a vacuum source for the process tool.
It is a still further advantage of the present invention to provide a system for firmly holding the port and pod doors together, which system is manually adjustable to accommodate for varying thicknesses of the port and pod doors.
It is another advantage of the present invention to provide a self-adjusting system in addition to or instead of the above-described manual adjustment system for accommodating variations in port and pod door thicknesses and for providing for engineering tolerances.
It is another advantage of the present invention to provide a system capable of providing a tighter coupling of a pod door to a port door than with conventional suction systems.
These and other advantages are provided by the present invention, which in preferred embodiments relates to mechanisms for pulling the pod door tightly against the port door as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool. In a preferred embodiment, the mechanism includes a latch key protruding outwardly from the outer surface of the port door. The latch key is provided to mate within a slot of a door latch assembly within the pod door. Once the latch key is properly seated within the slot, the latch key is rotated by mechanisms within the port door to decouple the pod door from the pod shell. Such rotation at the same time couples the pod door to the port door. According to preferred embodiments of the present invention, while the latch key rotates, it simultaneously moves in the rearward direction (i. e., back toward the port door) to thereby pull the pod door into a tight engagement with the port door.
In order to provide rearward translation of the latch key upon rotation, in a preferred embodiment, the latch key is affixed to a shaft including a rear threaded section. The threaded section is received within a threaded nut mounted within the port door so that the rotation of the latch key also moves the latch key rearward with respect to the nut.
The nut may be affixed within the port door by screws fitting within adjustment slots provided through the nut. While the latch key is in a stationary position, loosening of the screws from within the adjustment slots allows rotation of the nut to the extent of the slots, which in turn translates the latch key forward or rearward with respect to the outer surface of the port door. This adjustment mechanism allows the height of the latch key in front of the port door surface to be adjusted to accommodate variations in pod door thicknesses. Moreover, as a given rotation of the nut will result in a relatively small translation of the latch key, the nut adjustment assembly is capable of providing fine adjustment of the position of the latch key past the surface of the port door. The adjustment may be made more or less fine by decreasing or increasing, respectively, the pitch of the threads. Altering the thread pitch will also vary the translation of the latch key for a given rotation of the latch key.
In alternative embodiments of the present invention, the nut may be mounted on a spring loaded plate which allows self-adjustment of the axial position of the latch key. Such an alternative embodiment accommodates variations in port and pod door thicknesses, and also provides for engineering tolerances within the port and pod doors.
The latch key and slot that receives the latch key preferably include smooth surfaces to minimize particulate generation as the latch key rotates in the slot. However, in the event particulates are generated, they are trapped within the pod door. In a further alternative embodiment, the latch key may be provided with rollers which lie in contact with the walls of the slot as the latch key rotates to further prevent particulate generation.